What is IOTA?

A cryptocurrency with zero transaction fees, high scalability, and no mining centralization/resource expenditure

summary

IOTA is a cryptocurrency that goes "beyond blockchain" by using a newly invented protocol called the Tangle, instead of a blockchain.
To send a transaction, a user simply validates two other transactions on the Tangle/network. Since every participant is a transaction validator, there's no need for mining. This solves the major limitations of the blockchain such as fees, scalability, and centralization.
IOTA can be used for value transfers and data transfers alike, and intends to be the settlement layer for the internet-of-things

A bit more

IOTA is an open-source distributed ledger protocol launched in 2015 that goes 'beyond blockchain' through its core invention of the blockless 'Tangle'. The IOTA Tangle is a quantum-resistant Directed Acyclic Graph (DAG), whose digital currency has a fixed money supply with zero inflationary cost.

IOTA uniquely offers zero-fee transactions & no fixed limit on how many transactions can be confirmed per second.
Scaling limitations have been removed, since throughput grows in conjunction with activity; the more activity, the more transactions can be processed & the faster the network. Further, unlike blockchain architecture, IOTA has no separation between users and validators (miners / stakers); rather, validation is an intrinsic property of using the ledger, thus avoiding centralization.

IOTA is focused on being useful for the emerging machine-to-machine (m2m) economy of the Internet-of-Things (IoT), data integrity, micro / nano-payments, and other applications where a scalable decentralized system is warranted.